The advent of mobile communication technology has led to the proliferation of radiotelephones (also known as wireless telephone). Now, a person can carry with them a radiotelephone anywhere they go. A person can make a telephone call from almost anywhere to another person and can also receive a telephone call from anywhere.
Increasingly, radiotelephones are used in emergency situations to reach 911 emergency dispatchers. When handling 911 emergency telephone calls, it is important for the dispatchers to learn the physical location of callers. The physical location of a caller using an ordinary telephone connected to the Public Switched Telephone Network (PSTN) is easily obtainable, since the telephone is associated with a physical address stored in a database. There are no physical locations associated with radiotelephones.
It is not possible to determine the physical location of a radiotelephone, when the radiotelephone is capable of moving from one location to another. Radiotelephones are generally linked to the PSTN through a network of antennas, base stations, and mobile switching centers. Each radiotelephone is in communication with at least one antenna and switches from one antenna to another when it moves from one antenna cell to another antenna cell. Although each radiotelephone is associated with a telephone number, the telephone number does not translate into a fixed physical location. Furthermore, the knowledge of which antenna the radiotelephone is in communication with does not form a basis for determining the location of the radiotelephone itself.
Global Positioning System (GPS) has been employed to locate mobile devices. GPS is a satellite based system originally developed by the United States military for navigational purposes, but now it is also available for commercial purposes. GPS uses time of arrival (TOA) and triangulations for location calculation. GPS can provide accurate position information from a time-based signal received simultaneously from at least three satellites. A ground-based GPS receiver at or near the object to be located determines the difference between the time at which each satellite transmits a time signal and the time at which the signal is received and determines the object's location based on the time differentials. The accuracy provided by commercial GPS is within 100 meters. However, the employment of a GPS system is impractical in many applications. The signal power levels from the satellites are low and the GPS receiver requires a clear, line-of-sight path to at least three satellites above a horizon of about 60 degrees for effective operation. Moreover, inclement weather conditions, terrain features, and buildings further restrict the ability of the GPS receiver to determine its position. The GPS system also requires mobile devices to be equipped to link to earth satellites which makes these mobile devices more expensive.
Other techniques based upon signal strength and trilaterialization have been employed, but they are generally effective in line-of-sight conditions, such as rural settings. However, these techniques are not effective in dense urban areas.
In dense urban areas, radio waves reflect on buildings before reaching a receiving antenna on a mobile device, and the mobile device receives radio signals both directly from an emitting antenna and from reflections. This phenomenon is known as multipath signals, and it is well known in the wireless telephony art. The multipath phenomenon renders most analytical location computational techniques such as time-of-arrival (TOA) or time-difference-of-arrival (TDOA) substantially useless in urban areas.